Camera angle for 50% height hex grid
60°

Orthographic camera projects rays that are perpendicular to its plane. The length of a projected segment can be calculated using trigonometry.

In the first example of the figure below (left), a camera sees the segment from the top. The "projected" length of the segment is equal to its true length. The more the camera starts to face the horizon, the shorter the projection will be.



From the second example (right) we can easily figure out what is the general formula to get the length of the segment's projection upon the camera plane.

$$
P = R\cos{\theta}
$$

Where $P$ is the length of the projection, $R$ is the real length and $\theta$ is the angle.

In your case:

$$
P = \frac{R}{2}
$$

Then we can write the general equation as:

$$
\frac{R}{2} = R\cos{\theta}
$$

This holds when (one of the infinite possible solutions):

$$
\theta = \arccos{\frac{1}{2}} = 60
$$

In fact, the only combination of angles that makes a right-angled triangle whose one side length is half that of the hypotenuse is the 30-60 Triangle.



As the camera in its "angular resting" position is facing downward, the angle considered in the previous equations is the same angle you should put in the X rotation field.